1. Field of the Invention
The present invention relates to signal conditioning for digital wireline communications, e.g., echo cancellation, equalization, etc.
2. State of the Art
Echo cancellation, or echo attenuation, facilitates achievement of efficient, full-duplex data communications on two-wire channels. Various echo-cancellation techniques are known. FIG. 1 shows a block diagram of a widely known prior art xe2x80x9cadaptive hybridxe2x80x9d 100. Adaptive hybrid 100 is trained on the signal from the local transmitter received via port 103. This is usually done while the signal, normally received from the far end, is absent.
Filters 101 and 102 approximate the characteristics of nominal short and nominal long xe2x80x9cnear-endxe2x80x9d echo paths, respectively. Interpolator 104 is automatically adjusted in response to the operation of adaptive algorithm 105 in an attempt to provide an echo estimate to summer 106 that closely approximates the echo. Circuit 106 subtracts the estimated echo from one output of the fixed hybrid, and this output normally includes echo plus received signal.
The prior art approach of FIG. 1, however, provides little attenuation of some echoes. This limitation arises from the fact that a substantial percentage of echo paths (even near-end echo paths) differ widely from any characteristic that is capable of being provided as an interpolation between the two filters 101 and 102.
U.S. Pat. No. 5,204,854 describes an adaptive hybrid capable of achieving substantial echo attenuation for a wider variety of echo paths. This versatility, however, is achieved at the expense of substantial complexity.
There remains a need for an analog front end (AFE) that is simple in its realization yet highly effective in its attenuation of echo from a wide variety of echo paths. The present invention addresses this need.
The present invention, generally speaking, provides a digitally-tunable, echo-cancelling analog front end (AFE) for wireline digital communications. The analog front end is especially useful in a High-bit-rate Digital Subscriber Line (HDSL) or HDSL2 environment. An analog echo simulation path is provided capable of simulating echo from a wide variety of echo paths. Digitally controlled attenuators are provided in the transmission path and in the analog echo simulation path. Also provided is a digital-tunable equalizer stage. The equalizer stage is tuned to match the characteristics of the receive path. The same arrangement may be adapted for various DSL technologies, i.e., xDSL. There results an analog front end that is well-adapted to high-speed wireline communications.